Natural gas (NG) is a promising alternative fuel. Unfortunately, natural gas is often difficult to store, and natural gas storage requires extremely high pressures and/or at very low temperatures. To date, two techniques for NG storage are commercially used.
The first storage technique is to store Compressed Natural Gas (CNG) under extremely high pressure conditions such as, for example, 200-250 bars at normal ambient temperature. These pressures require specially designed reinforced tanks, which are bulky and heavy. Furthermore, gas compression requires an expensive and multi-stage high-pressure compression process. High-pressure vessels containing compressed natural gas are known to present a significant fire and/or detonation hazard.
Alternatively, Liquefied Natural Gas (LNG) is refrigerated to −161.5.degree. C. and stored at a more moderate pressure. This technique entails use of complex and expensive liquefaction equipment and thermos-like tanks as well as significant energy consumption (15-25% of the original gas energy content) for both liquefaction and regasification of the gas.
Both CNG and LNG technologies employ cylindrical or spherical storage tanks that lead to waste of space between neighboring vessels in arrays.
absorbed natural gas (ANG) is a promising alternative to these aforementioned technologies for natural gas storage since the same quantity of NG can be stored at much lower pressure (35-40 bars), at room temperature and in a thinner walled tank with a lower pressure rating. Furthermore, this method does not require expensive and cumbersome gas compression or liquefaction equipment, insulated tanks, etc. ANG tanks could have any configuration in contrast with exclusively cylindrical forms of NG high-pressure tanks. Thus, the tanks could be tailored to fit odd spaces, such as today's gasoline or diesel tanks in cars.
Salient features of ANG technology include high sorbent property and the thermal management system embedded within the vessel. ANG systems are thus characterized by the sorbent's ability for uptake and delivery of a maximum gas quantity. To date, sorbents based on activated carbon have shown promise. It is desirable to maximize the microporosity (fraction of the micropore volume) of the sorbent material so that the space occupied by the atoms of the microporous material and the space wasted by poor packing of the crystallites are both minimized.
For the specific case of adsorptive storage of natural gas, the efficacy of the absorbent in majority cases is measured by the adsorption capacity for gas per unit volume of absorbent at a specified pressure and room temperature. The adsorption capacity per unit volume of absorbent can be calculated by V.sub.v=(V.sub.w)(d), where Vw is the adsorption capacity of the material per unit mass of absorbent, and d is the density of the absorbent pellets. Upon compacting the material, the density d is increased, and so the adsorption capacity per unit volume V.sub.v also increases.
Primarily activated carbon sorbent is supplied in particulate matter such as powder or granulated powder form. Since volumetric performance is deciding factor in majority applications, especially for on-board fuel tanks, the sorbent needs to be compacted (immobilized).
Direct placement of the packed absorbent carbon into the storage vessel with a sufficient sorbent density has proven to be a formidable task. Briquetting, or immobilizing, the carbon was considered as an alternative. The advantages of briquetting were twofold Immobilized carbon would not settle and/or circulate in the storage vessel and would be less likely to be carried in the gas stream during discharge. Use of immobilized carbon would also allow the vessel to be packed more easily to a higher density than using just granular or powdered carbon.
Therefore, currently generally adopted tank design is based on multicell concept (FIG. 1), where the tank housing 1a includes of plurality of cells 6 and each cell in itself serves both as a pressurized vessel for the gas storing and as a container for sorbent briquettes 3a. This concept faces two serious difficulties.
The first difficulty is the complex design of the tank housing, whose manufacturing requires special profiling using expensive cumbersome equipment. The second is requirement that of high mechanical strength of the sorbent blocks.
In order to imbue sorbent blocks with the requisite mechanical strength, a significant quantity of binding medium is added to the sorbent. The use of a chemical binder, however, detracts from absorbent performance, because the binder tends to block methane access to the micropores of the carbon resulting in reduced storage and delivery. Furthermore, addition of binding material increases the size of the sorbent blocks without concomitantly increasing the amount of gas absorbent material within each block.
There is an ongoing need for absorbing gas storage systems including mechanically stable sorbent units within a storage tank. Current techniques do not provide viable solutions to this ongoing need, since on the one hand high-strength briquettes require strong binding material, and on other hand this binder concomitantly decreases sorbent uptake. Other technology for compensating for performance degradation associated with chemical binder use is very complex and expensive and still does not give desired results.
Furthermore, there is an ongoing need for absorbing gas storage systems designed to reduce the amount of time required to absorb and/or desorb adorable gas such as methane onto and/or from the sorbent, in order to reduce the time of filling the tank and/or removing the gas from the tank. It is noted that adsorption of methane gas onto the sorbent is an exothermic process, and thus as the gas is absorbed to the sorbent the ambient temperature within the storage tank increases, diminishing the rate of methane uptake by the sorbent. Similarly, desorption of gas reduces the ambient pressure and temperature within the tank, concomitantly increasing the time necessary to deliver absorbed gas from the tank. There is an ongoing need for systems and methods for removing heat from the tank during gas adsorption and for delivering heat to the tank during gas desorption. Preferably, the delivery and removal of heat should be relatively uniform throughout the tank in order to provide optimal conditions for all sorbent units within the vessel.
The following patents and published patent document, each of which are incorporated herein by reference, provide potentially relevant background art:
U.S. Pat. No. 6,019,823 discloses solid-phase physical sorbent medium holding absorbed fluid is provided in a cartridge, for use in a sorbent-based fluid storage and dispensing system;